1. Field of the Invention
This invention relates generally to methods, systems and apparatus for managing digital communications systems. More specifically, this invention relates to dynamically controlling system parameters that affect performance in communication systems such as DSL systems.
2. Description of Related Art
Digital subscriber line (DSL) technologies provide potentially large bandwidth for digital communication over existing telephone subscriber lines (referred to as loops and/or the copper plant). Telephone subscriber lines can provide this bandwidth despite their original design for only voice band analog communication. In particular, asymmetric DSL (ADSL) can adjust to the characteristics of the subscriber line by using a discrete multitone (DMT) line code that assigns a number of bits to each tone (or sub-carrier), which can be adjusted to channel conditions as determined during training and initialization of the modems (typically transceivers that function as both transmitters and receivers) at each end of the subscriber line.
Impulse noise, other noise and other sources of error can substantially impact the accuracy of data transmitted by ADSL and other communications systems. Various techniques have been developed for reducing, avoiding and/or repairing the damage done to data during transmission by such error. These error reduction/avoidance/repair techniques have performance costs for a communication system in which they are used. As is well known in the art, interleaving can reduce and/or eliminate adverse error effects by distributing errors generated by noise and other sources during transmission. Interleaving is a coding technique commonly used to increase the performance of transmission systems by decreasing errors in the system. Prior to transmission, interleaving rearranges transmission data to improve the error-correction performance of redundancy coding techniques by spreading errors out over more bytes of data.
The interleave “depth” is preferably defined conceptually and most generally as the distance between bits that originally were adjacent to one another. The interleave depth is altered by varying the distance between originally adjacent bits. Increasing the interleave depth improves the error correction capability of a given system. However, as discussed in more detail below, interleaving increases the transmission latency (that is, the time required for data to traverse the end-to-end transmission path) of the system. The interleave depth also may be a numerical value D that will be explained in more detail below.
One redundancy coding technique used in connection with interleaving is forward error correction (FEC) employing Reed-Solomon encoding, which is well known to those skilled in the art. FEC takes data to be transmitted by a user (referred to as “payload” data) and generates codewords that contain the “payload” data bytes and parity bytes (which also may be referred to as “redundancy” bytes in ADSL standards and various other publications), which assist a system receiver in checking for errors in transmitted data. These FEC codewords are inputs to a programmed transmitter interleaver.
Reed-Solomon codewords are made up of K data or information bytes (the user's payload data) and R parity bytes, totaling N bytes in each codeword (that is, K+R=N). Typically N must be 255 or less and DSL systems use only even numbers. The ADSL1 standard describes the methods used in FEC coding for that type of system and explains the use of FEC encoding and interleaving in detail. As noted in the ADSL1 standard, the FEC coder takes in S mux data frames of Kmux bytes of payload data and appends R FEC redundancy bytes to produce the FEC codeword of length NFEC=S×Kmux+R bytes. The FEC output data frames contain NFEC/S bytes, which is an integer.
In communication systems such as ADSL that use DMT techniques, each codeword may contain all or part of one or more DMT symbols. The variable S is used to refer to the number of DMT symbols present in each FEC codeword. Typically S=1 for fast transmission (low delay mode), since spanning more than one symbol introduces extra delay. In DSL, S can be a rational fraction as low as ⅓ (or effectively even lower in VDSL standards) and can be greater than 1. In some embodiments of the present invention, a modem may compute its S value based on other parameters.
The N, K, S and D parameters are directly specified by modem pairs in ADSL. In ADSL1 (or G.992.1/2), the downstream FEC settings for N, R, S, and D are determined by the transmitter (or ATU-C), while in ADSL2 (or G.992.3/4/5) the downstream FEC settings N, R, S and D and related parameters are determined by the downstream receiver (ATU-R) during loading. Upstream FEC settings are set by the ATU-C in both ADSL1 and ADSL2. The setting of S allows a modem the flexibility to match a the requested data rate with whatever internal overhead it determines it should use. If the modem also is told S, the modem might be put into a quandary of incompatible or otherwise conflicting requirements. Earlier systems have not used adaptive programming of the N, K and D parameters. While one system implemented by Alcatel currently allows 3 choices for D only, disadvantageously, the 2 choices useful for addressing impulse noise and the like have excessive delays and cause higher layer protocols to be less efficient.
Interleaving FEC codewords introduces a transmission delay (or “latency”), which can be a significant drawback in DSL systems. The latency associated with interleaving can constitute a significant portion of a system's overall latency. High latency can have a substantial negative impact on system performance, especially when the system is operating at high data transmission rates. The impact is especially pronounced for systems where many end-to-end transmissions are required to accomplish a task (for example, systems utilizing TCP/IP to send a large file). Accordingly, providers generally strive to minimize latency through their systems, though still tolerate it to gain the benefits of interleaving and offset adverse error effects. Therefore, it is desirable to minimize the interleaving necessary to achieve desired performance.
Some prior art systems use “adaptive” interleaving, which allows different interleaving depths to be applied to different transmissions. Adaptive interleavers are known to those skilled in the art. U.S. Pat. Nos. 4,901,319 and 6,546,509 describe adaptive interleaving systems, which adjust for various channel problems, noise and other sources of error. However, they require changes to the interleaving depths (denoted by the variable D) to be applied to different transmissions, which in turn alters the latency of the system.
Where the interleave depth is adaptively increased to reduce the error rate in earlier systems, the latency of the system increases, adversely affecting system performance and possibly leading to other performance-related problems. In addition, increasing interleave depth (and thus latency) may violate standards relating to prescribed latencies that are permitted for operation of a transceiver in an ADSL system. In some cases, increasing interleave depth even a small amount may compel large increases in latency due to the fact that some standards have discrete latency values. Therefore, a small, adaptive “bump up” in interleave depth by an adaptive interleaver may lead to a disproportionately large increase in latency, again a highly undesirable consequence of error correction.
ADSL standards prescribe some latencies. For example, in most implementations of ADSL1, the available latency delays typically are 4 ms and 24 ms (though occasionally a third option of 16 ms might be available) and are set by the DSLAM manufacturer's default for both upstream and downstream. In ADSL2 and ADSL2+ (G.992.5), the available latencies are set by the operator between 2 ms and 20 ms using the element management system (a data acquisition system, described in detail in G.997.1, that allows a service provider to implement control parameters such as the INP value and maximum interleaving delay), which conveys the preference to the line's modems, including the downstream receiver (ATU-R). That receiver selects the N, R and D parameters using a minimum impulse noise protection value or impulse strength indicator (called INP in G.992.3/4/5 and the related G.997.1), depending on vendor-imposed default options (sometimes referred to as “profiles”). The maximum interleaving delay and INP values in the G.992.3/4/5 and G.997.1 standards (which are incorporated herein in their entireties by reference for all purposes) however must be supplied upon initialization of the modem and there is no way to know any error measurements before data transmission. Emerging VDSL standards also use Reed Solomon codes and may have INP/Delay and/or N, R and D specification capability, depending on their evolution. Also, SHDSL standards may use bonding and Reed Solomon codes also and the same basic methods apply there to the setting of the N, R and D parameters or their equivalents like INP/Maximum Interleaving Delay. However, as can be seen from the available latency values in these systems, having to go to a higher latency as a result of adaptive interleaving and the like can mean a substantial increase in transmission delay.
Systems, methods and techniques that permit adjustment of a communications system to compensate for changing channel and other conditions, including noise effects, while minimizing and/or preserving the latency characteristics of the system would represent a significant advancement in the art. Moreover, maintaining a fixed latency delay for interleaving of data in an ADSL system, while adaptively adjusting the system to meet one or more error rate requirements would likewise constitute a significant advancement in the art.